A MULTI-STAGE PROCESS FOR PRODUCING A C2 TO C8 OLEFIN POLYMER COMPOSITION

Abstract

The present invention relates to a multi-stage process for producing a C.sub.2 to C.sub.8 olefin polymer composition in a process comprising at least two reactors, wherein a pre-polymerized solid Ziegler-Natta catalyst is prepared by carrying out an off-line pre-polymerization of a solid Ziegler-Natta catalyst component with a C.sub.2 to C.sub.4 olefin monomer before feeding to the polymerization process.

Claims

1: A continuous multi-stage process for producing a C.sub.2 to C.sub.8 olefin polymer composition in a process comprising at least two reactors, the process comprising the steps of: a) introducing a stream of a pre-polymerized solid Ziegler-Natta catalyst a monomer of C.sub.2 to C.sub.8 olefin into a first polymerization reactor, thereby producing a first C.sub.2 to C.sub.8 olefin polymer; b) withdrawing a stream comprising the first C.sub.2 to C.sub.8 olefin polymer from the first polymerization reactor and passing it into a second polymerization reactor; c) introducing a stream of a monomer of C.sub.2 to C.sub.8 olefin into the second polymerization reactor, thereby producing in the second polymerization a first polymer mixture comprising the first C.sub.2 to C.sub.8 olefin polymer and a second polymer of the C.sub.2 to C.sub.8 olefin monomer in the second polymerization reactor; and d) withdrawing a stream comprising the first polymer mixture from the second polymerization reactor; wherein the pre-polymerized solid Ziegler-Natta catalyst is prepared by off-line pre-polymerization of a solid Ziegler-Natta catalyst component with a C.sub.2 to C.sub.4 olefin monomer before feeding to the polymerization process, and wherein the pre-polymerized solid Ziegler-Natta catalyst has a particle size distribution span ((d.sub.90−d.sub.10)/d.sub.50) of below 1.5.

2: The continuous multi-stage process of according to claim 1, wherein the solid Ziegler-Natta catalyst component comprises: (a1) a compound of a transition metal (TM), which transition metal is selected from one of the groups 4 to 6 of the periodic table (IUPAC) (a2) a compound of a metal (M) which metal is selected from one of the groups 1 to 3 of the periodic table (IUPAC) (a3) optionally an internal donor (ID) and (a4) optionally a compound of a group 13 metal of the periodic table (IUPAC).

3: The continuous multi-stage process according to claim 1, wherein the solid Ziegler-Natta catalyst component is in the form of particles having a mean particle size ranging from 3 to 200 μm.

4: The continuous multi-stage process according to any one of claim 1, wherein the off-line catalyst pre-polymerization is carried out in the presence of a co-catalyst (Co) and optionally in the presence of an external electron donor (ED); and/or wherein the off-line catalyst pre-polymerization is carried out with a total residence time of 5 to 55 minutes.

5: The continuous multi-stage process according to claim 1, wherein the off-line catalyst pre-polymerization degree is from 0.1 to 50 g.sub.polymer/g.sub.cat.

6: The continuous multi-stage process according to claim 1, wherein the off-line catalyst pre-polymerization is carried out in a medium, wherein the medium is an oil or a hydrocarbon solvent.

7: The continuous multi-stage process according to claim 1, wherein the off-line catalyst pre-polymerization is carried out at a temperature in the range from 0 to 50° C.

8: The continuous multi-stage process according to claim 1, wherein pre-polymerized solid Ziegler-Natta catalyst has a particle size distribution span ((d.sub.90−d.sub.10)/d.sub.50) of below 1.2.

9: The continuous multi-stage process according to claim 1, wherein the first polymerization reactor in step a) is a loop reactor.

10: The continuous multi-stage process according to claim 1, wherein the process comprises at least one gas phase reactor.

11: The continuous multi-stage process according to claim 1, wherein the process comprises a further step e) of introducing the stream of step d) and a stream of a monomer of C.sub.2 to C.sub.8 olefin to a third polymerization reactor, wherein a second polymer mixture is produced.

12: The continuous multi-stage process according to claim 11, wherein the process further comprises a step f) of withdrawing a stream comprising the second polymer mixture from the third polymerization reactor.

13: The continuous multi-stage process according to claim 11, wherein the process comprises two loop reactors and one gas phase reactor.

14: The continuous multi-stage process according to claim 11, wherein the process comprises one loop reactor and two gas phase reactors.

15: The continuous multi-stage process according to claim 12, wherein the process further comprises a step g) of introducing the second polymer mixture to a fourth polymerization reactor together with additional monomers selected from C.sub.2 to C.sub.8 olefins, whereby a third polymer mixture is produced.

Description

FIGURES

[0120] FIG. 1 refers to the particle size distribution of polyethylene obtained by using the inventive and comparative Ziegler-Natta catalysts of Examples 5-7.

[0121] In the following the present invention is further illustrated by means of examples.

EXAMPLES

1. Measuring Methods

[0122] The following definitions of terms and determination methods apply for the above general description of the invention as well as to the below examples unless otherwise defined. Particle size distribution [mass percent] and particle sizes were determined by laser diffraction measurements by Coulter LS 200.

[0123] Particle size distribution (PSD) defined by SPAN:

[00001]$\mathrm{Span}=\frac{D90-D10}{D50}$

[0124] The particle size and particle size distribution is a measure for the size of the particles. The D-values (D.sub.10 (or d.sub.10), D.sub.50 (or d.sub.50) and D.sub.90 (or d.sub.90)) represent the intercepts for 10%, 50% and 90% of the cumulative mass of sample. The D-values can be thought of as the diameter of the sphere which divides the sample's mass into a specified percentage when the particles are arranged on an ascending mass basis. For example the D.sub.10 is the diameter at which 10% of the sample's mass is comprised of particles with a diameter less than this value. The D.sub.50 is the diameter of the particle where 50% of a sample's mass is smaller than and 50% of a sample's mass is larger than this value. The D.sub.90 is the diameter at which 90% of the sample's mass is comprised of particles with a diameter less than this value. The D.sub.50 value is also called median particle size. From laser diffraction measurements according to ASTM 13320-1 the volumetric D-values are obtained, based on the volume distribution.

[0125] The distribution width or span of the particle size distribution is calculated from the D-values D.sub.10, D.sub.50 and D.sub.90 according to the below formula:

[00002]$\mathrm{Span}=\frac{D90-D10}{D50}$

[0126] The mean particle size corresponds to the average particle size. From laser diffraction measurements according to ASTM 13320-1 the volume based mean particle size is obtained and calculated as follows:

[00003]${\overline{D}}_{pq}^{\left(p-q\right)}=\frac{.Math.D_i^p}{.Math.D_i^q}$

wherein D=the average or mean particle size [0127] (p−q)=the algebraic power of D.sub.pq, whereby p>q [0128] D.sub.i=the diameter of the ith particle [0129] Σ=the summation of D.sub.ip or D.sub.iq representing all particles in the sample

[0130] Only in symmetric particle size distributions the mean particle size and the median particle size D.sub.50 have the same value.

[0131] Unless specifically otherwise defined, the percentage numbers used in the text below refer to percentage by weight.

2. Examples

[0132] Solid Ziegler-Natta Catalyst Preparation

[0133] The catalyst used for the following examples was prepared according to the catalyst preparation example of WO2017207493.

[0134] Raw Materials

[0135] The standard 10 and 25 wt % TEA (triethyl aluminum) solutions in heptane were prepared by dilution of 100% TEA-S from Chemtura.

[0136] MgCl.sub.2*3EtOH carriers were received from GRACE. 2,2-Di(2-tetrahydrofuryl)propane (DTHFP) was supplied by TCI EUROPE N.V. as a mixture (1:1) of diastereomers (D,L-(rac)-DTHFP and meso-DTHFP.

[0137] TiCl.sub.4 was supplied by Aldrich (Metallic impurities<1000 ppm, Metals analysis>99.9%).

[0138] Preparation of Pre-Treated Support Material

[0139] A jacketed 160 dm.sup.3 stainless steel reactor equipped with a helical mixing element was pressurized with N.sub.2 to 2.0 barg and depressurized down to 0.2 barg until the O.sub.2 level was less than 3 ppm (barg is a measure of over-pressure, i.e. pressure above atmospheric pressure). The vessel was then charged with heptane (20.5 kg) and 2,2-di(tetrahydrofuryl)propane (0.512 kg; 2.78 mol; DTHFP). The obtained mixture was stirred for 20 min at 40 rpm. The MgCl.sub.2-3EtOH carrier (6.5 kg; DTHFP/Mg=0.1 mol/mol; 27.2 mol of Mg; Mg 10.18 wt.-%, d.sub.10=9.5 μm, d.sub.50=17.3 μm and d.sub.10=28.5 μm, granular shaped) was added to the reactor with stirring. This suspension was cooled to approximately −20° C. and 33 wt.-% solution of triethylaluminum (29.4 kg; 85.0 mol of Al; Al/EtOH=1.0 mol/mol) in heptane was added in aliquots during 2.5 h time while keeping the temperature below 10° C. After the TEA addition, the reaction mixture was gradually heated to 80° C. over a period of 2.4 h and kept at this temperature for additional 20 min at 40 rpm. The suspension was allowed to settle for 10 min, and the mother liquor was removed through a μm filter net in the bottom of the reactor during 15 min. The vessel was charged with warm toluene (43 kg) and then stirred at 40 rpm for 38 min at 55 to 70° C. The suspension was allowed to settle for 10 min at 50 to 55° C. and the liquid removed through a 10 μm filter net in the bottom of the reactor during 15 min.

[0140] Catalyst Preparation

[0141] The vessel containing the pre-treated support material was charged with toluene (43 kg) and then cooled to approximately 30° C. Neat TiCl.sub.4 (5.17 kg, 27.5 mol; Ti/Mg=1.0 mol/mol) was added. The obtained suspension was heated to approximately 90° C. over a period of 2 h and kept at this temperature for one additional hour with stirring at 40 rpm. The suspension was allowed to settle for 10 min at approximately 90° C. and the mother liquor was removed through a 10 μm filter net in the bottom of the reactor during 15 min. The obtained solid material was washed twice with toluene (43 kg each) at ˜90° C. and once with heptane (34 kg) at ˜40° C. All three of these washing steps used the same sequence of events: addition of preheated (90 or 40° C.) solvent, then stirring at 40 rpm for 30 min, allowing the solid to settle for 10 min, and then removal of liquid through a 10 μm filter net in the bottom of the reactor during 15 min.

[0142] The obtained catalyst was mixed with 20 kg of white oil and dried 4 h at 40-50° C. with nitrogen flow (2 kg/h) and vacuum (−1 barg). The catalyst was taken out from the reactor and the reactor was flushed with another 20 kg of oil and taken out to the same drum. The dry catalyst yield was 3.60 kg (82.2% based on Mg).

[0143] Off-Line Pre-Polymerized Ziegler-Natta Catalyst

[0144] Diluted Ziegler-Natta catalyst oil slurry (37.37 kg containing 4.9 wt.-% of solid Ziegler-Natta catalyst as described above was added to the reactor at 20° C. followed by 33 wt.-% solution of TEAL in heptane (0.87 kg; Al/Ti=1.0 mol/mol). Off-line pre-polymerization was initiated almost immediately after TEAL addition (stirring time ˜5 min) by continuous addition of propylene at 20-25° C. A target pre-polymerization degree was set equal to 2 g polymer/g.sub.cat and the desired degree of pre-polymerization was reached after 5 h 10 min (Example 7). The pressure was released and the reactor was flushed five times with nitrogen and then dried under vacuum for 1 h. Dried off-line pre-polymerized Ziegler-Natta catalyst in oil was taken out into a catalyst drum. The yield was 42.97 kg containing 13.9 wt.-% of off-line pre-polymerized Ziegler-Natta catalyst with pre-polymerization degree of 2 polymer g/g.sub.cat. The off-line pre-polymerized Ziegler-Natta catalyst had a particle size distribution of d.sub.10=10.0 μm, d.sub.50=30.1 μm and d.sub.90=47.3 μm (SPAN of 1.24).

[0145] Examples 1 to 4 below are simulated examples using Ziegler-Natta model catalysts having the particle size distribution of d.sub.10=10 μm, d.sub.50=20 μm and d.sub.10=30 μm (SPAN of 1).

Example 1—Comparative

[0146] Model Catalyst mimicking the catalyst prepared according to Catalyst preparation example above (without off-line pre-polymerization) was polymerized in a continuous PE pre-polymerization reactor (on-line pre-polymerization). The residence time was 5 minutes, the temperature was 50° C. and the production rate was 3 Kg PE/g.sub.cat/h. This example represents a case where the polymer particles stayed very short time in the on-line pre-polymerization reactor.

[0147] Subsequently, the polymer material from the on-line pre-polymerization step transferred to the first loop reactor and subsequently to the second loop reactor. The total residence time in the two loop reactors connected in series was equal to 60 mins, the temperature was equal to 90° C. in both loop reactors and the pressure was 63.5 bar in the first loop reactor and 62.5 in the second loop reactor. After a high pressure flashing step, the particulate material was transferred to the gas phase reactor (GPR) where 5% mol 1-butene was also added. The temperature in the GPR was 85° C., the pressure was 20 bar and several superficial gas velocity values selected between the range of 0.1 m/s to 0.5 m/s, having a nominal value equal to 0.4 m/s.

[0148] In GPR the particle overheating (i.e., the temperature difference between the particles surface and the gas phase, expressed as DT) was estimated for various superficial gas velocity values (SGV) showing the areas of good (SGV=0.5 m/s) and poor fluidization conditions (SGV=0.1 m/s).

[0149] As it is well understood in the art the superficial gas velocity denotes the velocity of the gas in an empty construction. Thus, the superficial gas velocity within the middle zone is the volumetric flow rate of the gas (in m.sup.3/s) divided by the cross-sectional area of the middle zone (in m.sup.2) and the area occupied by the particles is thus neglected.

[0150] Depending on the operating conditions (e.g., superficial gas velocity, mixing intensity, comonomer partial pressure, polymer particle size, etc.) the heat transfer limitation can lead to particle overheating, softening and, thus, agglomeration. In order to avoid the particle overheating and agglomeration.

[0151] The particle overheating (ΔTov) is defined as the temperature difference between the particles (T.sub.PP) and the gas phase (T.sub.S)

ΔTov=T.sub.PP−T.sub.S,

wherein

ΔTov=Q.sub.g/(hA.sub.PP),

[0152] with

[0153] Q.sub.g being the heat that is produced due to polymerization (it is calculated based on the polymerization kinetics, or reactor productivity),

[0154] h being the external heat transfer coefficient and A.sub.PP is the external surface of the polymer particles.

h=(k.sub.gNu)/D.sub.PP,

[0155] wherein

[0156] k.sub.g is the thermal conductivity of the gaseous mixture (transport property),

[0157] D.sub.PP is the diameter of the polymer particles and

[0158] Nu is a dimensionless number that is defined as the ratio of convective to conductive heat transfer across external boundary layer of the polymer particles and it very much depends on the superficial gas velocity (operating condition).

[0159] In the above calculation all temperatures are given in ° C.

[0160] More information regarding the calculation can be found in a well-established procedure that is described in articles: Gas-Phase Olefin Polymerization in the Presence of Supported and Self-Supported Ziegler-Natta Catalysts, V. Kanellopoulos, B. Gustafsson and C. Kiparissides, (2008), Macromolecular Reaction Engineering 2(3), pp.: 240-252 and in Comprehensive Analysis of Single-Particle Growth in Heterogeneous Olefin Polymerization: The Random-Pore Polymeric Flow Model, V. Kanellopoulos, G. Dompazis, B. Gustafsson and C. Kiparissides, (2004), Industrial & Engineering Chemistry Research 43(17), pp.: 5166-5180.

[0161] The results are shown in Table 1 below. It can be gathered that high particle overheating can be observed, especially for the large size catalyst particles, which can potentially result in severe operability issues.

TABLE-US-00001 TABLE 1 Tendency for agglomeration in GPR for various SGV and catalyst sizes (t.sub.off-line pre-polymerization = 0 mins, t.sub.continuous prepol. = 5 mins). d10 = 10 μm d50 = 20 μm d90 = 30 μm SGV = 0.1 m/s SGV = 0.5 m/s SGV = 0.1 m/s SGV = 0.5 m/s SGV = 0.1 m/s SGV = 0.5 m/s   DT = 6° C.   DT = 4° C.   DT = 25° C.   DT = 15° C.   DT = 37° C.   DT = 22° C.

Example 2—Inventive

[0162] The polymerization series described in Example 1 was repeated with the only difference that an off-line pre-polymerization step was employed where all catalyst particles experienced the same residence time of 30 minutes and off-line pre-polymerization degree of 20 polymer g/g.sub.cat. The results are shown in Table 2 below. It can be gathered that by employing an off-line pre-polymerization step, the tendency of particle overheating in the gas phase is lower compared to the case where a continuous on-line pre-polymerization without an off-line pre-polymerization step was employed (as in Example 1).

TABLE-US-00002 TABLE 2 Tendency for agglomeration in GPR for various SGV and catalyst sizes (t.sub.off-line prepol = 30 mins, t.sub.continuous prepol.. = 5 mins). d.sub.10 = 10 μm d.sub.50 = 20 μm d.sub.90 = 30 μm SGV = 0.1 m/s SGV = 0.5 m/s SGV = 0.1 m/s SGV = 0.5 m/s SGV = 0.1 m/s SGV = 0.5m   DT = 4° C.   DT = 3° C.   DT = 14° C.   DT = 9° C.   DT = 18° C.   DT = 10° C.

Example 3—Comparative

[0163] Example 1 (Comparative) was repeated with the only difference being the residence time in the continuous on-line pre-polymerization step that is 30 min. The particle size distribution of the particles exiting the continuous pre-polymerizer (d.sub.p) without a preceding off-line pre-polymerization step was measured. The results are shown in Table 3 below. It can be gathered that a broad PSD is obtained (i.e., span=1.66).

TABLE-US-00003 TABLE 3 Catalyst and particle size distribution in pre-polymerization without an off-line pre- polymerization step 1 (t.sub.off-line prepol = 0 mins, t.sub.continuous prepol. = 30 mins). Size (μm) Width (d.sub.90-d.sub.10) Span (d.sub.90-d.sub.10)/d.sub.50 d.sub.c 10  20  30  20 1.0 d.sub.p 65 238 440 375 1.66 d.sub.c = particle size of the model catalyst before the off-line pre-polymerization step

Example 4—Inventive

[0164] Example 3 (Comparative) was repeated with the differences being that the catalyst was off-line pre-polymerized as in Example 2 before entering the continuous pre-polymerization step, where the residence time was also 30 min. The particle size distribution of the particles exiting the continuous pre-polymerizer (d.sub.p) is shown. The results are shown in Table 4 below. It can be gathered that a narrow PSD is obtained (span 1.04).

TABLE-US-00004 TABLE 4 Catalyst and particle size in pre-polymerization with an off-line pre-polymerization step (t.sub.off-line prepol = 30 mins, t.sub.continuous prepol. = 30 mins). Size (μm) Width (d.sub.90-d.sub.10) Span (d.sub.90-(d.sub.10)/d.sub.50) d.sub.c 10  20  30  20 1.0 d.sub.p 65 130 200 135 1.04 d.sub.c = particle size of the model catalyst before the off-line pre-polymerization step

Examples 5 and 6 (Comparatives) and Example 7 (Inventive)

[0165] The Ziegler-Natta catalyst described in the “Off-line pre-polymerized Ziegler-Natta catalyst” above was used in Example 7 (Inventive), and the same catalyst without off-line pre-polymerization step was used in Examples 5 and 6 (comparatives) (see Table 5). Catalysts were employed in a multi-stage polymerization process, consisting of a continuous pre-polymerization reactor and two slurry-phase loop polymerization reactors followed by a gas phase reactor connected in series. Polymerization in a continuous pre-polymerization reactor was conducted at a temperature of 60° C., and under pressure of 58 bar. Subsequently, the polymer material was transferred to the first loop reactor (T=95° C., and P=56 bar) and then to the second loop reactor (T=95° C. and P=54 bar). After that, flashing of the unreacted components and solvent carried out in a high pressure flash tank and the particulate material was transferred to a gas-solid fluidized bed reactor (GPR), where it was polymerized at temperature of 85° C., at 20 bar pressure using a superficial gas velocity of 0.3 m/s. All the reactors operating conditions, the various components feed streams rates, the production splits and representative properties of the produced polymers in each polymerization step of the multi-stage process are depicted in Table 5. The resulted particle size distribution curves are illustrated in FIG. 1 and the D.sub.10, D.sub.50 and D.sub.90 values of the corresponding particle size distributions are reported in Table 5. As it can be seen, the final polymer particles produced by the off-line pre-polymerized Ziegler-Natta catalyst (Example 7, reported in Table 5) exhibit a much narrower particle size distribution (Span=1.6) compared to those ones produced by the Ziegler-Natta catalyst without any off-line pre-polymerization step (Span=2.11 and Span=2.18) of the prior art (Comparative Examples 5 and 6). Results are disclosed in Table 5.

TABLE-US-00005 TABLE 5 Summary of polymerization conditions Example 5 and 6 (Comparative) and Example 7 (Inventive). Example 7 Inventive (IE7) Example 5 Example 6 Off-line pre- Comparative Comparative polymerized (CE5) (CE6) Ziegler- Catalyst Ziegler-Natta catalyst Natta catalyst Pre-polymerization reactor Temp. (° C.) 60 60 60 Press. (bar) 58 58 58 C2 (kg/h) 2.0 2.0 2.0 H2 (g/h) 2.0 5.0 2.0 Split % 1.3 1.3 1.3 First loop reactor Temp. (° C.) 95 95 95 Press. (bar) 56 56 56 C2 conc. (mol-%) 4.1 3.9 3.5 H2/C2 ratio 529 469 572.5 Split % 16.4 16.4 16.4 MFR2 (g/10 min) 210 121 260 Second loop reactor Temp. (° C.) 95 95 95 Press. (bar) 54 54 54 C2 conc. (mol-%) 3.1 3.8 4.5 H2/C2 ratio 500 428 481 Split % 37 38 37 MFR2 (g/10 min) 360 216 440 GPR Temp. (° C.) 85 85 85 Press. (bar) 20 20 20 H2/C2 ratio 38 26 28 C6/C2 ratio 24.2 24.2 26.6 Split % 45.4 44.8 45.1 Powder Density (kg/m.sup.3) 954 953 955 MFRS (g/10 min) 0.18 0.13 0.21 MFR21 (g/10 min) 7.0 4.6 7.24 D10 (μm) 76 85 145 D50 (μm) 246 257 287 D90 (μm) 596 645 605 SPAN 2.11 2.18 1.60